<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="lib/jquery-3.3.1.min.js"></script>
    <style>
        .current{
            color:red;
        }
    </style>
</head>

<body>
    <p>hello world</p>
    <button id="btn">btn</button>
    <script>
        var one=$("p").hasClass("current");
        console.log(one);
        $("#btn").click(function(){
            if($("p").hasClass("current")){
                $("p").removeClass("current");
            }else{
                $("p").addClass("current");
            }
        })
    </script>
</body>

</html>